There is known a current measuring apparatus using the Faraday effect by which a plane of polarization of light rotates due to the action of a magnetic field. As examples of current measuring apparatuses of this type, reflection type current measuring apparatuses are disclosed in Japanese Patent Application Public Disclosure No. H10-319051 and Japanese Patent Application Public Disclosure No. 2000-292459.
This type of current measuring apparatus is advantageous in that it is not affected by electromagnetic noise, and detects only a current flowing through a portion of a conductor encircled with an optical fiber, and is not affected by a current in a portion of the conductor outside the optical fiber. Therefore, it has been proposed to use this type of current measuring apparatus for a gas-insulated switch gear, or for identifying a section of occurrence of a short circuit or ground fault, monitoring a change of electricity supply and demand due to new market participants, or achieving efficient distribution to match supply to demand.
In the reflection type current measuring apparatuses disclosed in the above-mentioned patent documents, an optical fiber sensor is extended or looped around a conductor through which a current to be measured flows. Measurement is made with respect to an angle of Faraday rotation, under a magnetic field of the current to be measured, of linearly polarized light which is emitted into one end of the optical fiber sensor and reflected at an opposite end of the optical fiber sensor.
FIG. 11 shows an essential part of a conventional reflection type current measuring apparatus. This current measuring apparatus comprises a reflection type optical fiber sensor 2 extended or looped around a conductor 1 through which a current to be measured flows.
A light-transmitting ferromagnetic Faraday element 3 capable of magnetic saturation is disposed on a side of an input end of the optical fiber sensor 2. The ferromagnetic Faraday element 3 is adapted to rotate a plane of polarization of linearly polarized light through 22.5°. A light-transmitting birefringent member 4 is disposed on a side of a forward end of the ferromagnetic Faraday element 3. The birefringent member 4 is adapted to separate the light emitted from the optical fiber sensor 2 into an ordinary ray and an extraordinary ray that are orthogonal to each other, and to guide these rays to light-receiving elements.
However, this current measuring apparatus is technically disadvantageous as explained below.
When the current measuring apparatuses of the above-mentioned patent documents are actually used for measurement, the light must be converged at a core portion of the optical fiber. Therefore, for example, as shown in FIG. 11, lenses 7 are individually disposed in an area between the optical fiber sensor 2 and the ferromagnetic Faraday element 3 and an area between the birefringent member 4 and an optical fiber 5 for introducing the linearly polarized light into the birefringent member 4, or an optical fiber 6 for guiding the extraordinary ray after the separation.
With this arrangement, however, a separation distance between the ordinary ray and the extraordinary ray emerging from the birefringent member 4 is relatively narrow, so it is extremely difficult to provide two lenses 7 between the birefringent member 4 and the optical fibers 5 and 6 in a parallel relationship.
If the separation distance is increased sufficiently to accommodate the lenses 7, since the separation distance and the thickness of the birefringent member 4 are proportional to each other, the thickness of the birefringent member 4 becomes large, so that an entire structure of the current measuring apparatus becomes large.
Further, the structure shown in FIG. 11 requires a large number of components, which complicates the structure. Aligning the lenses 7 and the optical fibers is time-consuming, and a large number of operations are required for assembly, which results in a high cost of manufacture.
In the apparatus of Japanese Patent Application Public Disclosure No. H10-319051, a plane-parallel plate made of a single-axis birefringent crystal is disposed in a current detection unit. However, the structure of this apparatus does not allow an operation for assembly such that the polarization preserving optical fiber 5 and the optical fiber 6 are fixed relative to each other to thereby form a subassembly, and connected to the lenses. Therefore, each of the optical fibers must be independently fixed. Therefore, with respect to each optical fiber, a space corresponding to the volume of a fixing member and a space for an assembly operation are required. Therefore, the distance between the polarization preserving optical fiber and the optical fiber becomes large, so the current detection unit becomes large.
As a countermeasure, an arrangement shown in FIG. 12 is considered. In this arrangement, a Rochon prism 4a is used as a birefringent member, and a single lens 9 is disposed between the Rochon prism 4a and the optical fibers 5 and 6.
Further, an arrangement can be made as shown in FIG. 13, in which a wedge-shaped prism 4b made of a single-axis birefringent crystal is used as a birefringent member and the single lens 9 is provided between the wedge-shaped prism 4b and the optical fibers 5 and 6. In either of the arrangements shown in FIGS. 12 and 13, a reduction in the number of components can be achieved.
However, in these arrangements, the respective optical paths of the ordinary ray and the extraordinary ray emerging from the Rochon prism 4a or the wedge-shaped prism 4b are not parallel. When the ordinary and extraordinary rays enter the lens 9 in this state, the ordinary ray and the extraordinary ray must be made parallel after passage through the lens 9. Therefore, the structure of the lens 9 becomes complicated and delicate adjustment is required, thus making it difficult to obtain a desired effect.
On the other hand, in Japanese Patent Application Public Disclosure No. 2000-292459, the current measuring apparatus is simplified in structure by connection of a current detection unit and a photoelectric converter through a single optical fiber. However, it is impossible to compensate for a variation in the measured value of the current due to the temperature characteristic of a 22.5° Faraday element which is disposed in the current detection unit. That is, the light is received by the photoelectric converter in a state such that an angle of rotation of the linearly polarized light in a current detection optical fiber, which is obtained by the Faraday effect, is combined with a temperature-induced change in the angle of rotation of a plane of polarization of the light in the 22.5° Faraday element. Therefore, the temperature-induced change in the angle of rotation of the plane of polarization of the light in the 22.5° Faraday element cannot be separated from the rotation produced by the Faraday effect. Thus, there is no means to provide an inexpensive current measuring apparatus which comprises a small-size current detection unit and which efficiently receives the light to be measured that is emitted from the current detection unit, while compensating for a variation in the measured value of the current due to the temperature characteristic of the 22.5° Faraday element.
Next, referring to FIG. 14, description is made with regard to a conventional transmission type current measuring apparatus. In a transmission type current measuring apparatus, an optical fiber sensor 200 is extended or looped around a conductor through which a current to be measured flows. One end of the optical fiber sensor 200 is connected to a thin type polarizer 202, and the other end of the optical fiber sensor 200 is connected to a polarized-light splitting unit 204. The polarizer 202 receives random light emitted from a light source (not shown) and transmits, to the optical fiber sensor 200, only linearly polarized light consisting of a wave oscillating in the same direction. The linearly polarized light, when passed through the optical fiber sensor 200, is subject to a magnetic field produced by the current to be measured, and a plane of polarization of the linearly polarized light is rotated through a predetermined angle that is proportional to a magnitude of the magnetic field. The linearly polarized light in this state is emitted from the other end of the optical fiber sensor 200 and enters the polarized-light splitting unit 204, in which the linearly polarized light is split into an ordinary ray and an extraordinary ray. The ordinary ray is outputted to a first optical fiber 206, and the extraordinary ray is outputted to a second optical fiber 208. The ordinary ray from the first optical fiber 206 and the extraordinary ray from the second optical fiber 208 are outputted to a photoelectric converter (not shown). In the photoelectric converter, the ordinary ray and the extraordinary ray are respectively converted into electrical values, which are in turn supplied to a signal processing circuit (not shown). Based on these electrical values, an angle of Faraday rotation is determined, and a magnitude of the current to be measured is finally determined.
The polarized-light splitting unit 204 comprises a birefringent member 210 for splitting the linearly polarized light into the ordinary ray and the extraordinary ray, a lens 212 for guiding the linearly polarized light emitted from the output end of the optical fiber sensor 200 to the birefringent member 210, a lens 214 for guiding the ordinary ray emitted from the birefringent member 210 to the first optical fiber 206, and an optical path shift prism 216 and a lens 218 for guiding the extraordinary ray emitted from the birefringent member 210 to the second optical fiber 208. A crystal axis of the polarizer 202 and a crystal axis of the birefringent member 210 are angularly displaced at 45° relative to each other, so that the birefringent member 210 is capable of splitting the linearly polarized light from the optical fiber sensor 200 into an ordinary ray and an extraordinary ray that are orthogonal to each other.
Thus, in the transmission type optical fiber sensor, it is required to provide the two lenses 214 and 218 between the birefringent member 210 and the optical fiber sensors 206 and 208. When the separation distance is made large, the optical path shift prism 216 is also required. Consequently, an entire structure of the current measuring apparatus is large.
The present invention has been made in view of the above-described disadvantages. It is an object of the present invention to provide a current measuring apparatus which is reduced in size due to a reduction in the number of components, and which can be easily assembled.